The present invention relates to a shadow mask for a cathode ray tube (CRT) and, more particularly, to a shadow mask which has a sufficient degree of structural strength.
Generally, CRTs are provided with a color selection shadow mask for ensuring the correct landing of electron beams on a phosphor screen.
FIG. 12 is a cross sectional view of a CRT with a shadow mask according to a prior art, and FIG. 13 is a perspective view of the shadow mask shown in FIG. 12.
As shown in FIG. 12, the CRT includes a faceplate panel 3 with an inner phosphor screen 1, and a funnel 5 and a neck 7 sequentially connected to the panel 3. An electron gun 9 is fitted within the neck 7 to emit R, G and B electron beams. A shadow mask 11 is mounted within the panel 3 while facing the phosphor screen 1 at a close distance. The shadow mask 11 is fixed to the panel 3 by interposing a mask frame 13. The shadow mask 11 has a plurality of beam-guide apertures 11a that make it possible for each of the R, G and B electron beams to strike only its intended phosphor on the phosphor screen 1.
As shown in FIG. 13, the shadow mask 11 is formed with a beam-guide portion 11b where the aforementioned apertures 11a are placed, a non-opening portion surrounding the beam-guide portion 11b, and a side wall 11c bent from the non-opening portion at an angle. The side wall 11c of the shadow mask 11 is welded to the mask frame 13 such that the beam-guide portion 11b faces the phosphor screen 1 at a close distance. The distance between the beam-guide portion 11b of the shadow mask 11 and the phosphor screen 1 is usually called the xe2x80x9cQ valuexe2x80x9d.
In order to constantly maintain the Q value, the beam-guide portion 11b of the shadow mask 11 is curved with a curvature radius corresponding to that of the internal surface of the panel 3 where the phosphor screen 1 is formed. In this connection, as the panel 3 becomes flatter and flatter, the curvature radius of the effective screen portion 11b should be increased as much,.
When the curvature radius of the beam-guide portion 11b increases to a large extent, its mechanical strength is seriously deteriorated. Furthermore, considering that the shadow mask 11 is formed with an extremely thin metal plate where numerous apertures 11a are located, the beam-guide portion 11b of the shadow mask 11 exhibits extremely weak structural strength.
In the above structure, the beam-guide portion 11b of the shadow mask 11 is liable to suffer deformation even at minimum degrees of vibration or shock. In this case, the apertures 11a formed at the beam-guide portion 11b deviate from their proper positions so that the electron beams cannot strike correct phosphors on the phosphor screen 1, and this deviation results in poor picture images.
It is an object of the present invention to provide a shadow mask for a CRT which has a sufficient degree of structural strength, and produces good picture images.
This and other objects may be achieved by a shadow mask for a CRT having a faceplate panel with an inner phosphor screen. The shadow mask includes a front surface and a side wall. The front surface is formed with a beam-guide portion having a plurality of apertures, and a non-opening portion surrounding the beam-guide portion. The side wall is bent from the non-opening portion at an angle, and fixed to the panel via a mask frame. The front surface of the shadow mask has a predetermined waved pattern in at least one direction.
The predetermined wave pattern of the front surface is structured to satisfy the following condition:       -    0.03     less than             Δ      ⁢              xe2x80x83            ⁢      Q        Q     less than       +    0.03  
where Q indicates the average distance between the front surface and the phosphor screen, and xcex94Q indicates the difference between the actual distance of the aperture of the shadow mask to the phosphor screen and the average distance between the front surface and the phosphor screen.